The invention relates to a process for producing fluorescent or phosphorescent pigments coated with glass.
Luminescent pigments with fluorescent or phosphorescent properties are to be protected by coating with glass against greying, to which they are susceptible under service conditions after a certain time, for example as a result of photochemical reactions.
Coating of individual pigment crystals with glass according to a sol-gel process has heretofore been known in the art. The German Specification No. 2,747,509 discloses such a process for coating zinc sulphide or zinc/cadmium sulphide based fluorescent or phosphorescent luminescent pigment particles with a glass consisting of:
50 to 90 percent by weight of SiO.sub.2 and 10 to 50 percent by weight of B.sub.2 O.sub.3 and/or Al.sub.2 O.sub.3 and/or MgO and/or CaO and/or ZnO and/or Li.sub.2 O and/or Na.sub.2 O and/or K.sub.2 O. PA1 a) mixing the luminescent pigment into an alcoholic metal alcoholate solution, which may contain water, containing metal oxides in a mixture ratio of 50 to 90 percent by weight of SiO.sub.2 and 10 to 50 percent by weight of B.sub.2 O.sub.3 and/or Al.sub.2 O.sub.3 and/or MgO and/or CaO and/or ZnO and/or Li.sub.2 O and/or Na.sub.2 O and/or K.sub.2 O; PA1 b) allowing said mixture to jelly by diffusing moisture into the mixture; and PA1 c) heating said jellied mixture to a temperature of 100.degree. C. to 800.degree. C., preferably of 200.degree. C. to 500.degree. C.
The procedure according to the invention comprises the following steps:
In the process, a fine powder of pigment crystals individually coated with a thin glass layer is obtained. The glass portion amounts to approximately 0.1 to 0.01 percent of the pigment particles by weight.
There are disadvantages, however, to this procedure. It has been found that the glass coated pigments still grey significantly after only one to two weeks of exposure. Further, it has been found that the glass coating produced has very low mechanical strength. For this reason, the pigment particles produced by the sol-gel method are not suitable for use in mechanically loaded phosphorescent coatings, for example, on concrete blocks or concrete slabs which are laid on carriageways.
Luminescent tiles are also known, in which a layer of luminescent pigments is covered by a transparent glaze.
The object of the invention is a process of the kind mentioned in the preamble with which it is possible to produce a glass coating of the pigments which protects the pigment in large measure against weathering effects and has a high mechanical strength.
This object is achieved according to the invention by the features set out in the characterizing part of claim 1.
Suitable developments of the process for the production, and suitable processes for the application, of such glass-coated pigments are contained in the sub-claims.